The present invention relates to a patient monitoring system and more particularly to a vital signs monitor and an alarm system incorporated therein.
A need exists in the health care area for a low cost, reliable method of measuring patient respiration characteristics on a continuous basis and alarming when the measured parameters exceed certain preset thresholds.
Many situations exist where this monitoring capability can be life saving. Examples being (1) intraoperative monitoring in heavy sedation with or without local anesthesia (ultrasonic ovum retrieval, herniorrhaphy, breast biopsy, etc.), (2) emergency room monitoring, (3) recovery room monitoring, (4) intensive care unit monitoring, (5) patient transportation from an accident scene to a hospital, from a nursing home to the hospital, and finally during transportation of the patient after general anesthesia from the operating room to the recovery room.
The candidates for the monitoring are patients with or requiring (a) arrival in the recovery room after an operation which required heavy sedation with or without local anesthesia, (b) obstructive pulmonary disorders, (c) cardio vascular instabilities due to trauma, (d) post surgery anesthesia care, (e) high medicinal loading, (f) evaluation for commitment to ventilator support machines, (g) weaning from ventilators and (h) known predisposition to apnea episodes.
The monitor will alarm when threshold levels are exceeded relating to (1) tidal volume (TV) (volume of exhaled gas), (2) respiration rate (RR), (3) minute volume (RR.times.TV), (4) approximate oxygen consumed per breath, (5) approximate end tidal carbon dioxide, and (6) combinations of the above.
Additionally, the monitor will continuously display all analog values and the monitor will also provide a display of flow rate showing the dynamic movement of gases through the airway.
The monitoring of these vital signs is critical to the emergency and critical care patient. Changes in pulmonary function can signal life threatening events, respiratory arrest, apnea, hypoxemia and brain death.
Present technology relies on continual visual observation of a patient in a potentially threatening state, as well as an indirect measure of pulmonary or ventilatory function.
Visual observations have problems associated with it that are due to the inability of humans to provide continuous surveillance of a situation for a long period of time without loss of attention. This attention span is further minimized when duties are concurrently expanded to other tasks and patients.
Periodic blood gas analysis has value, but it is time consuming, and in acute changes of ventilation the time to act is very short. The blood gas analysis will show a trend in a patient's condition, but use of a mass spectrometer for the gas analysis has limited value especially in acute changes of ventilation and, thus, will not save a patient who experiences a rapid change in vital signs.
Continuous use of pulse oxyimeters will monitor arterial oxygen that is hemoglobin bound and provide readings of percentage saturation of hemoglobin (red blood cell) oxygen. This method has two major draw backs.
(1) When any patient motion exists at the sensor site the readings are unreliable due to the fact that the machine must necessarily look for readings that have noise artifacts and reject them when they are not compatible with prior averages. This causes present readings to be late. Since diminished pulmonary function can cause irreversible brain damage in 2 to 4 minutes, late readings are intolerable. Additionally, the patient at high risk frequently is being transported (motion) during the trauma, the trauma in and of itself can induce physical responses (motion).
(2) The rate that arterial oxygenation levels decline in the present of pulmonary arrest is dependant on several factors, namely, cardiac output, patient age, medicinal loading, physical condition, and other patient pathologies including poor peripheral perfusion. A common observation is complete pulmonary arrest, and oxygen arterial saturation levels of 80 to 95% several minutes after the pulmonary arrest.
Another method of assessing pulmonary function is to quantitize end tidal carbon dioxide. By measuring expired carbon dioxide as a percentage in terms of its partial pressure in the expired or exhaled gases, a measure of the rate of metabolism can be estimated since essentially all exhaled carbon dioxide which is the product of metabolism is excreted from the lungs through the mouth. Presently end tidal carbon dioxide is measured only in general anesthesia use. Even in the event that end tidal carbon dioxide was used universally, it is subject to problems associated with low tidal volume and the associated rebreathing of anatomical and mechanical dead space gases causing sufficient rises in the partial carbon dioxide pressures of exhaled gases. Also, at low respiration rates due to the solubility of carbon dioxide, it readily dissolves in blood gases by passing through tissue lining at very low partial pressures, allowing for false negative conditions to exist.
Further in an emergency environment false positives can occur as a result of esophageal carbon dioxide from stomach contents, for instance, beer and carbonated sodas.
A device is available that monitors the presence or absence of respiration by monitoring tidal flows and alarms at a preset time on the cessation of respiration. Such a device is disclosed in U.S. Pat. No. 5,063,938. The device disclosed in this patent does not provide quantitative information on patient tidal volumes.